Surgical instrument system

ABSTRACT

A surgical instrument system includes a tool ( 2 ) including an elongate shaft which defines the tool axis. The shaft bears a plurality of marker rings ( 10, 11, 12 ) arranged in a predetermined pattern on the surface of the shaft so that they extend around the shaft axis, the marker rings identifying the tool. The system includes a device ( 14 ) for receiving signals from the rings, and a data processor ( 16 ) for analysing the signal from the rings and generating information relating to the identity of the tool.

The present invention relates to a surgical instrument system. Thesystem finds particular application in the field of image guidedsurgery.

Some surgical techniques can be automated using computer apparatus. Forexample, it is known to use computers to generate signals to control theposition of surgical instruments. These techniques can be used forexample in conjunction with image data relating to the patient, forexample as to the location, size, orientation etc of tissue on which theprocedure is to be performed. In particular, if the location of thesurgical instrument can be determined relative to the tissue on whichthe procedure is to be performed, it is possible in some circumstancesfor manipulation of the instrument to be controlled throughout theprocedure using signals generated by a computer.

Reliable performance of such automated surgical procedures requires thatthe position of the instrument should be determined accurately. This canbe done using one or more detectors which can detect signals from theinstrument. For example, markers can be provided on the instrument fromwhich a signal can be detected. The markers can be passive, for examplemade from reflective material, or they can be active, for examplecomprising two or more devices which emit radiation (such as lightemitting diodes).

In the case of a surgical tool which is rotated about its axis such as adrill bit, it is known to provide markers for monitoring the position ofthe drive unit for the tool. This has can give rise to inaccuracies whenthe tool is long, when the end of the rotating part (whose location islikely to be of greatest significance) is spaced apart from the markerson the drive unit, for example due to flexing of the tool.

The present invention provides a surgical instrument system in whichring markers are placed on the shaft of a tool.

Accordingly, in one aspect, the invention provides a surgical instrumentsystem, which comprises:

-   -   a. a tool including an elongate shaft which defines the tool        axis, the shaft bearing a plurality of marker rings arranged in        a predetermined pattern on the surface of the shaft so that they        extend around the shaft axis, the marker rings identifying the        tool,    -   b. a device for receiving signals from the rings,    -   c. a data processor for analysing the signal from the rings and        generating information relating to the identity of the tool.

The instrument system of the present invention has the advantage that itenables identification signal data to be obtained from a tool withoutthe need for markers to be provided on a handle or other control part.The signal data can therefore be obtained from a part of the instrumentwhich can be close to the end of the tool which is in contact with apatient's tissue; the signal data therefore comes from the part of thesystem of the invention which whose identity is particularly importantin controlling the outcome of the procedure.

The signal data can also provide information as to the position andorientation of the tool relative to the receiving device. Providing thisdata based on markings on the tool has the advantage that the signaldata is not subject to inaccuracies that can be associated with markerslocated on a handle or drive unit, for example arising from flexing ofthe tool.

The nature of the marker rings on the shaft of the tool are such thatthey will appear when viewed by the receiving device as rectangles whenthe tool is viewed from one side, perpendicular to a plane containingthe axis of the tool, especially when the shaft is rotating about itsaxis. When the device is viewed obliquely from one side, the appearanceof the rings will deviate from being exactly rectangular in that thesides of the rectangular will appear curved.

Preferably, the marker rings will extend continuously around the shaftof the tool. However, for some applications, the marker rings can have asmall break provided in them, provided that the break does not interferesignificantly with determining the location and spacing of the edges ofthe ring.

Preferably, the planes defined by the axially spaced edges of each ringare parallel to one another and perpendicular to the axis of the shaft.The rings will then appear as rectangles when the tool is viewed fromone side perpendicular to a plane containing the axis of the tool. Whenthe tool is viewed obliquely from one side, the edges of the rings willappear curved and will be continuously equidistant (parallel) around theaxis of the tool. This has the advantage that it can facilitate analysisof the position data relating to the rings by the computer, even whenthe edges of the rings do not appear to the receiving device to bestraight.

Determining the position of the tool will often require accuratedetermination of the relative positions of the rings when viewed by thereceiving device. Steps which can be included in the determination oftheir relative positions can include one or more of:

-   -   a. locating the axis of the shaft, which will generally extend        along a line which is equidistant from the opposite edges        (extending parallel to the shaft axis) of the rings, and    -   b. locating the centre line of each ring, which will intersect        the axis of the shaft at a point which is equidistant from the        opposite edges (extending perpendicular to the shaft axis) of        the rings.

Preferably, the data processor is programmed to perform the steps of:

-   -   a. locating the axis of the shaft, which will generally extend        along a line which is equidistant from the opposite edges        (extending parallel to the shaft axis) of the rings, and    -   b. locating the centre line of each ring, which will intersect        the axis of the shaft at a point which is equidistant from the        opposite edges (extending perpendicular to the shaft axis) of        the rings.

Preferably, the data processor is programmed to perform the steps of:

-   -   a. identifying three generally rectangular areas which represent        the rings on the shaft when the shaft is viewed from one side,    -   b. determining the location of a line on each of the rectangular        areas which represents the midpoint of each area, measured        parallel to the axis of the tool,    -   c. determining the angle between the lines on adjacent ones of        the rectangles.

Especially when the rings are narrow, it might be the relative positionsof the rings can be determined with sufficient accuracy without locatingtheir centre lines (step (b) above), thereby reducing the errors inposition.

The steps for determining the relative positions of the rings whenviewed by the receiving device can be performed by the data processor,for example by determining the coordinates of the relevant edges, andthen interpolating to locate the relevant centre line.

Preferably, the system includes at least three marker rings on thesurface of the shaft. Preferably, the distance between a first ring anda second ring which is adjacent to the first ring is the same as thedistance between the said second ring and a third ring which is adjacentto the second ring on the opposite side of the second ring from thefirst ring.

The rings on the shaft can be used to provide information about thetool, including for example being used to identify the tool. This can beachieved by providing the rings in a predetermined arrangement which canbe identified by the system. For example, the rings can be provided onthe shaft as a bar code as is known from other contexts. The dataprocessor can be provided with data a look up table relating tools ofthe general type being used, so that data that is obtained by analysingthe signal from the rings can be compared with data in the look up tableto enable the tool to be identified. For example, the tool can beidentified by reference to the width of the bands or the spacing betweenadjacent bands or both. The dimensions (width, spacing etc) of the ringswill be selected according to the resolution of the receiving device,the likelihood of obstruction to clear inspection of the rings (forexample because of body fluids in the vicinity of the tool when in use),the range of distances from the receiving device to the tool etc.

The width of the rings, measured along the axis of the shaft, will beselected according to factors such as the nature of the receiving deviceand the signals which are received by the device from the rings, and thedistance between the tool and the receiving device. The rings should besufficiently wide that they can be seen by the receiving device.However, it will often be preferred for the width of the rings to bekept to a minimum, consistent with the requirement for visibility, sothat inaccuracies in determining the relative positions of the rings areminimised.

Preferably, the width of each ring is less than about 10 mm, for exampleless than about 5 mm. Generally the width of each ring will be at leastabout 3 mm.

Preferably, the spacing between adjacent edges of adjacent rings will beat least about 1 mm, more preferably at least about 3 mm, especially atleast about 5 mm. Preferably, the spacing is not more than about 20 mm,more preferably not more than about 10 mm.

The rings can have a surface which is a contrasting colour or texturefrom the remainder of the shaft on which the rings are arranged. Forexample, the shaft might have a generally dull surface and the ringsmight have a relatively smooth surface, especially so that it is glossy.The shaft might have a generally dark surface, and the rings might berelatively bright. Accordingly, it is preferred that the marker ringsare more reflective than the surface of the shaft on which they arearranged.

The shaft of the tool will generally be made from a metal. Metals whichare frequently used in the manufacture of surgical instruments,especially cutting tools, include certain stainless steels. The ringswill often be applied using a different material, for example apolymeric material or an inorganic material. The nature of the materialof the rings will be selected according to factors such as (a) itsreflective or other properties which are necessary for the rings to bedetected by the receiving device, and (b) the conditions to which therings will be exposed prior to, during, and after use of the instrument.When the rings are applied to the shaft of the tool and are intended toremain their permanently, they might be required to withstand theconditions to which the tool is exposed when the tool is prepared foruse, for example by exposure to elevated temperature and pressure.Certain pigments (which might be based on polymeric materials) can besuitable. For example, a pigment can be included in a curable polymericmaterial such as an epoxy resin. Different factors might apply whenrings are applied to the shaft of the tool for the purpose of a singleprocedure. For example, the rings can be provided as markings on apolymeric sleeve, for example provided by an ink. A preferredconstruction of sleeve might comprise a sleeve formed from a transparentpolymeric material, with a plurality of contrasting bands located on theinternal surface of the sleeve.

The materials used in the rings will depend on the nature of thereceiving device. When the device is a camera which can observe visibleradiation, the rings should contrast visibly with the surface of theshaft on which they are arranged. When the device receives data which isnot in the visible region of the electromagnetic spectrum, differentring materials might be used. For example, if the device receivesinfrared or ultraviolet radiation, the rings should reflect radiationfrom an infrared source or an ultraviolet light source respectively

The system of the invention can include two (or more) receiving deviceswhich are spaced apart, for receiving stereoscopic signals from therings on the tool, and possibly three or more receiving devices foraccurate determination of the position and, especially, the orientationof a tool. The use of a plurality of appropriately positioned receivingdevices allows orientation to be determined by identifying the effect ofparallax on the observed spacing between adjacent rings. The use of tworeceiving devices also allows position and orientation of a tool to bemonitored with reduced risk of obstruction of the line of sight.

The positions of the receiving devices will be determined in order tooptimise the range of motion of the tool without obstruction of the lineof sight of the tool using the devices. Preferably, the general lines ofsight for the receiving devices are not parallel to one another so thatthe angle between the lines of sight is generally less than 150°, forexample less than about 120°. The said angle will generally be more thanabout 30°, for example more than about 60°. Appropriate selection of theangle can help to provide stereoscopic determination of the orientationand position of the tool.

The rings can be provided on the shaft on a sleeve which is applied tothe surface of the shaft. For example, the sleeve can be made from amaterial which allows the sleeve to shrink when subjected to anappropriate treatment such as heating or exposure to a liquid. The ringscan instead be provided directly on the surface of the shaft, forexample by the application of a layer of contrasting material onto thesurface of the shaft, or by engraving the surface of the shaft, or by acombination of the two.

It will generally be necessary to register the tool in the coordinatespace defined with respect to the receiving device. The registrationprocess can have the purpose of any of (a) determining the nature of theinstrument, and (b) determining the spatial relationship between therings on the shaft and an operating part (for example the end wherecutting teeth are located) of the tool. The nature of the registrationprocess will be different if the rings are applied to the tool on aseparate sleeve compared with a tool on which the rings are provided aspermanent features. The nature of registration procedures in imageguided surgery is generally established.

The tool can be a cutting tool, for example a drill or a reamer. Thetool can have other functions, for example as another type of finishersuch as a polisher.

Preferably, the system of the invention includes a drive unit forimparting rotational motion to the tool. For example, the drive unit canbe of a kind that is used conventionally to impart rotational motion toa tool such as a drill bit. The invention has particular advantages whenused in this way, in that the markers are visible on the shaft of thetool throughout its rotation. However, the invention has relatedadvantages when the tool does not rotate about its shaft because themarkers can be observed irrespective of the orientation of the tool

The dimensions of the shaft will vary according to the nature of thetool and its intended application. The shaft will often have a circularcross-section. Its diameter might be less than about 25 mm, for exampleless than about 20 mm, possibly less than about 10 mm, even less thanabout 5 mm. The length of the shaft will generally be kept to a minimum,consistent with being able to use the tool at a desired location.However, it will be appreciated that the shaft might be long for someapplications, and it is there that the advantages that are afforded bythe present invention become particularly significant. Preferably, therings on the shaft will be positioned as close as reasonably possible tothe end of the tool that is remote from the drive unit.

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 is a schematic representation of a system according to thepresent invention.

FIG. 2 is a side view of a cutting tool for use in the system of theinvention.

Referring to the drawings, FIG. 1 shows a surgical instrument systemwhich comprises a rotatable drill bit 2 having an elongate shaft. Theshaft has a cutting portion 4 and a smooth surface portion 6. The drillbit is made from a surgical grade stainless steel. The diameter of theshaft is 6 mm in the illustrated embodiment, although it will beappreciated that the drill bit could have a size which is bigger thanthis or smaller than this.

The system includes a drive unit 8 which the drill bit 2 fits into, andwhich causes the drill bit to rotate about the axis defined by theshaft.

The smooth surface portion 6 of the shaft has first, second and thirdmarker rings 10, 11, 12 on its surface. Each of the rings has a width(measured along the axis of the shaft) of 4 mm. The distance betweenadjacent edges of adjacent rings is 3 mm so that the centre to centrespacing of the rings is 7 mm. The rings on the surface of the shaft areprovided by a curable polymeric material such as a epoxy materialcontaining an appropriate pigment.

The system includes two cameras 14, positioned with respect to the driveunit and cutting tool so that the rings on the shaft of the cutting toolare visible throughout use of the cutting tool in a surgical procedure.The pigment should be such that reflected light from the surface of thedrill bit enables a sharp contrast to be observed between the rings andthe regions of the shaft between the rings.

The system includes a data processor 16 which can analyse the signalsreceived from the cameras. The signals are analysed to determine thespacing between adjacent pairs of the rings, that is between the firstand second rings, and between the second and third rings, as viewed byeach camera. Determining the spacing involves (a) locating the axis ofthe shaft, which will generally extend along a line which is equidistantfrom the opposite edges (extending parallel to the shaft axis) of therings, and (b) locating the centre line of each ring, which willintersect the axis of the shaft at a point which is equidistant from theopposite edges (extending perpendicular to the shaft axis) of the rings.If the shaft of the tool is arranged so that it extends preciselyperpendicular to a line which extends between the camera and the rings,the measured distances between the first and second rings, and betweenthe second and third rings, respectively, will be the same. If the shaftof the tool is arranged so that the shaft is not exactly perpendicularto the line extending between the camera and the rings, the measureddistances between the first and second rings, and between the second andthird rings, respectively, will be different. For example when the thirdring is further from the camera than the first ring, the measureddistance between the first and second rings will be greater than themeasured distance between the second and third rings. The orientation ofthe tool relative to the camera (which can be considered in terms of theangle between the shaft axis and the line extending between the cameraand the rings) can be determined by simple trigonometry from knowledgeof the apparent distances as determined from the receiving device andthe actual distances as known from the registration of the tool.

1. A surgical instrument system, which comprises: a. a tool including anelongate shaft which defines the tool axis, the shaft bearing aplurality of marker rings arranged in a predetermined pattern on thesurface of the shaft so that they extend around the shaft axis, themarker rings identifying the tool, b. a device for receiving signalsfrom the rings, c. a data processor for analysing the signal from therings and generating information relating to the identity of the tool.2. A system as claimed in claim 1, in which the marker rings are morereflective than the surface of the shaft on which they are arranged. 3.A system as claimed in claim 1, in which there are at least three markerrings.
 4. A system as claimed in claim 3, in which the distance betweena first ring and a second ring which is adjacent to the first ring isthe same as the distance between the said second ring and a third ringwhich is adjacent to the second ring on the opposite side of the secondring from the first ring.
 5. A system as claimed in claim 1, whichincludes a drive unit for imparting rotational motion to the tool.
 6. Asystem as claimed in claim 1, which comprises two receiving deviceswhich are spaced apart for receiving stereoscopic signals from the ringson the tool.
 7. A system as claimed in claim 1, in which the planesdefined by the axially spaced edges of each ring are parallel to oneanother and perpendicular to the axis of the shaft.
 8. A system asclaimed in claim 1, in which the rings are marked on a sleeve which isfitted to the surface of the tool.
 9. A system as claimed in claim 1, inwhich the tool is a cutting tool.
 10. A system as claimed in claim 1, inwhich the data processor is programmed to perform the steps of: a.locating the axis of the shaft, which will generally extend along a linewhich is equidistant from the opposite edges (extending parallel to theshaft axis) of the rings, and b. locating the centre line of each ring,which will intersect the axis of the shaft at a point which isequidistant from the opposite edges (extending perpendicular to theshaft axis) of the rings.
 11. A system as claimed in claim 1, in whichthe data processor is programmed to perform the steps of: a. identifyingthree generally rectangular areas which represent the rings on the shaftwhen the shaft is viewed from one side, b. determining the location of aline on each of the rectangular areas which represents the midpoint ofeach area, measured parallel to the axis of the tool, c. determining theangle between the lines on adjacent ones of the rectangles.